This invention relates to a branching device interposed in a half duplex transmission line. More specifically, the present invention relates to a branching device which is used to connect branch lines of communication module appliances disposed at individual machining stations to a main transmission line.
Conventionally, a communication system is known wherein sequence controllers and computers disposed at individual machining stations in a factory, are connected to a main transmission line to effect mutual communications between them. Robots, numerically controlled machine and so forth disposed at the individual machining stations are controlled by way of the communication system.
In the communication system, branch lines of individual communication module appliances are connected to the main transmission line. Accordingly, when a failure such as breaking of a wire, short-circuiting or the like occurs at one of the branch lines to the communication module appliances or at the main transmission line, it is difficult to find out the location of the failure or to determine a manner of the failure. When a failure of breaking of a wire occurs, it is possible to separate the station in failure to allow each of the other stations free from failure to continue its processing by connecting the main transmission line into a loop. However, when a failure of short-circuiting occurs, communications between all of the communication module appliances are disabled. Therefore, in order to find out a location of a failure of short-circuiting, it is necessary to divide the transmission line in many sections and check them in an order. However, this is a very difficult operation.
Accordingly, once the system fails, the entire system must be stopped, and much time is required for discovery of the failure and elimination of the trouble.
Besides, there is a possibility that, when one of the communication module appliances connected to the main transmission line by way of the branch lines is damaged by an overvoltage or an overcurrent, such damage may not be restricted to part of the system but may reach the entire system.
Some of the inventors of the present invention have proposed a multiplex communication system for a sequence controller (U.S. patent application Ser. No. 07/235,498) to solve such a problem. In the present invention, moreover, reduction of time required for elimination of a trouble upon failure of a system is anticipated.